1. Technical Field
The present invention relates to a method of manufacturing a multi-layer semiconductor nanoparticle that emits fluorescence of a narrow wavelength width, and the multi-layer semiconductor nanoparticle manufactured by the method.
2. Background Art
Semiconductor nanoparticles of a grain size of 10 nm or less are located in the transition region between bulk semiconductor crystals and molecules. Their physicochemical properties are therefore different from both bulk semiconductor crystals and molecules. In this region, the energy gap of a semiconductor nanoparticle increases as its grain size decreases, due to the appearance of a quantum-size effect. In addition, the degeneracy of the energy band that is observed in bulk semiconductors is removed and the orbits are dispersed. As a result, a lower-end of the conduction band is shifted to the negative side and an upper-end of the valence band is shifted to the positive side.
Semiconductor nanoparticles can be easily prepared by dissolving equimolar amounts of precursors of Cd and X (X being S, Se or Te). This is also true for their manufacture using ZnS, ZnSe, HgS, HgSe, PbS, or PbSe, for example.
However, the semiconductor nanoparticles obtained by the above method exhibit a wide grain-size distribution and therefore cannot provide the full advantage of the properties of semiconductor nanoparticles. Attempts have been made to attain a monodisperse distribution by using chemical techniques to precisely separate the semiconductor nanoparticles having a wide grain-size distribution immediately after preparation into individual grain sizes and extract only those semiconductor nanoparticles of a particular grain size. The attempts that have been reported so far include an electrophoresis separation method that utilizes variation in the surface charge of a nanoparticle depending on the grain size, an exclusion chromatography that takes advantage of differences in retention time due to different grain sizes, a size-selective precipitation method utilizing differences in dispersibility into an organic solvent due to different grain sizes, and a size-selective photocorrosion.
Semiconductor nanoparticles obtained by these methods exhibit a spectrum with a relatively narrow wavelength-width peak, however, even in such a monodisperse state, the light-emission characteristics of the crystal are not quite satisfactory. This is presumably due to the presence of the energy level of a defect site on the particle surface in the forbidden band exhibited by the nanoparticle. Thus, by removing the energy band in the forbidden band, the light-emission characteristics of the semiconductor nanoparticle can be improved. Various methods have been attempted so far to remove the energy band in the forbidden band.
In one method, for example, the semiconductor nanoparticle surface is coated with an organic component such as tri-n-octyl phosphine (TOP) and tri-n-octyl phosphine oxide (TOPO). In another method, the semiconductor nanoparticle surface is coated with an inorganic component such as CdX and ZnX.
The semiconductor nanoparticle manufacturing method employing size-selective photocorrosion makes it possible to manufacture not only large quantities of semiconductor nanoparticles very easily but also manufacture semiconductor nanoparticles having a very narrow grain-size distribution. Further, by coating the surface of semiconductor nanoparticles with an inorganic component to remove the energy band in the forbidden band, multi-layer semiconductor nanoparticles that show a spectrum with a narrow wavelength-width peak can be prepared. However, it has been difficult to combine the former and the latter techniques into one process because of the difference in the stabilizer used.